1. Field of Invention
The present invention relates to a modular system adapted to provide ballast to hold a roofing membrane in position, and to retain a relatively large volume of water on the roof to assist in control of water runoff from the building.
2. Description of Prior Art
Roof membranes of the type commonly specified on commercial or industrial buildings are held in place with weight resting on the membrane. The membrane is held in place with ballast materials such as round river rocks or weighted pavers made from concrete or other dense materials. Minimum ballast is commonly designated at approximately 11 lbs/sq-ft to hold the membrane in place. Additional ballast is specified as required such as to meet wind uplift resistance requirements.
It is often desirable and advantageous to retain water on the roof of such industrial and commercial buildings during rainfall. Retaining water on the roof reduces peak load in storm sewers, and smoothes out the peak sewer drainage load over a period of time. In certain locations such as densely developed cities controlling water runoff from a building may affect the footprint size of the building which may be built on a particular lot. Retaining water on the roof of a building also permits the retained water to evaporate from the roof over a long period of time, contributing to cooling of the roof surface and the air above roof. This cooling reduces the urban heat-island problem that is often associated with large buildings in densely developed areas.
Prior roof membrane ballast arrangements suffer from several drawbacks and disadvantages. Disadvantage of round river rocks is that they are relatively expensive, and it is difficult to maintain and repair the roof membrane because removal of the loose ballast material is a labor intensive, difficult and expensive process. It is also difficult to safely walk on a roof that is covered with loose round river rocks due to movement of rocks. And neither pavers nor round river rocks have any significant water retention capability.
Another type of system that is capable of providing ballast for a roof membrane is a green roof. Green roofs are provided as either modular or non-modular systems. The modular green roofs include modules that are positioned across the membrane in side-by-side relation and filled with growing medium and vegetation. Non-modular green roofs are established with fibrous mats laid over the membrane, such as with a base support mat and a top mat in which the vegetation grows.
One advantage of green roof systems is their ability to retain water during rainfall. However, green roofs are relative expensive, and they do not provide easily predictable rainfall retention characteristics. Variations in growing medium, plant type, root structure, and other covering result in variation of water retention capability. Thus, the water retention capability of a green roof can not be predetermined with repeatable specificity. And to prevent root rot, green roofs are intentionally designed to retain only limited volumes of water for any extended period of time. In modular systems, this is accomplished with drainage holes near the base of the modules below the growing medium, and in non-modular systems, this is accomplished through designation of the fiber structure and material of the base and top mats.
Consequently, there is a need for an improved ballast system for roofing membranes that addresses the above-identified drawbacks and disadvantages of prior ballast systems. In addition, there is a need for a ballast system that is adapted to retain predictable volumes of water on a roof during and after a rainfall.